Contact printing or exposure is a traditional graphic arts procedure for generating same size, high resolution reproductions of line and halftone film images. This technique ensures faithful original film image reproduction by placing the original film image in intimate contact with a receiving film or paper emulsion and, in the presence of a vacuum to hold them in intimate contact throughout the exposing process, exposing through the original film image to the receiving film or paper by means of a point or reflected light source. Typically, these images are of a relatively large size in order to meet the needs of the graphic arts industry and thus require large format originals and receiving films and the equipment necessary to handle such sized films.
Traditional silver halide contact films have sufficient photographic visible light sensitivity that the contacting exposure can be carried out with a simple, low cost, low energy tungsten light source. However, because of the sensitivity of these films to visible light, the contact exposure must be carried out in a darkroom environment. This causes a major inconvenience for the operator and results in a loss of productivity in the graphic arts production process.
In an attempt to improve graphic arts contact exposure productivity, film manufacturers have developed normal roomlight handling contact films. These films are photographically very insensitive to visible light, but have a relatively high UV sensitivity (although, in fact, having a relatively low UV sensitivity compared to the UV sensitivities of darkroom-handled film). These roomlight-handled films have improved the productivity of graphic arts contact exposure operations by eliminating the bottlenecks of the darkroom environment. However, they have also increased the use of high intensity, expensive, high UV output light sources. Metal halide light sources are a common type of UV light source used to expose these films.
One of the more important aspects of the graphic arts contact exposing or printing process is the need for accurately reproducing very fine lines and halftone dots. This has resulted in the requirement that the exposing light be collimated at the exposing plane so that such lines and dots can be reproduced accurately without undercutting or spreading and changing the size of the reproduced line or dot and the resulting image. Adequate light collimation has been achieved in the past by using a point light source mounted at a sufficiently great distance from the exposing plane that the light is substantially collimated by the time it reaches the exposure plane.
Given the large exposing plane area necessary for graphic arts purposes, the need for a high degree of light collimation to achieve accurate image reproduction, and the relative low photographic UV sensitivities of roomlight handling contact films, a high energy, expensive metal halide source has heretofore been required to expose these films. These high intensity UV light sources have required that the operator be shielded from the light source to avoid exposure to harmful UV radiation. Examples of such arrangements are illustrated in U.S. Pat. Nos. 4,029,404, 4,316,669, and 4,437,759. As a result, the total volume of space required for the graphic arts contact exposing set-ups of the prior art has been large and, since most printing shops have more than one of these exposing set-ups, the total volume of space required is significant.
It has been found that one of the limiting factors to increasing the productivity of graphic arts contact printers is that contact printers of the prior art have required that the exposure of the film wait until the film has been drawn into intimate contact throughout the entire extent of the interface with the original before the exposure can commence. A contact printer which permits the exposure to begin before the entire film/original laminate has been evacuated has been developed which employs a film cover sheet which is deployed over the original/film laminate and through which the film is exposed. It has been found that with use this cover sheet can become scratched and dirty, interferring with the accurate reproduction of graphic arts images.